Study On Formation Mechanism And Control Technologies Of Trichloroacetonitrile,Disinfection By-product From L-Tryptophan In Chlorination

Chlorination has always been a common disinfection method used in the drinking water treatment process.However,the disinfection by-products(DBPs)are produced in this process,which have the highly carcinogenic,teratogenic and mutagenic effect on the human health.Therefore,it has a very important practical significance to study the formation mechanism and control methods of these disinfection by-products to ensure the drinking water safety and human health.The detection method of trichloroacetonitrile(TCAN)was established by liquid-liquid extraction and gas chromatography(LLE-GC)method using methyl tertiary butyl ether(MTBE)as extractant and1,2-dibromopropane as internal standard,respectively.The target and internal standard were well separated by this method.And the measurement method for TCAN was highly accurate with recovery rate between 98.9% and 102.4%,relative standard deviation of 5.37%~14%and minimum detection limit less than 6.79?g/L.TCAN was generated when the precursor L-tryptophan reacted with the disinfectant sodium hypochlorite and with the increasing reaction time,TCAN production was raised rapidly at first,and then was reduced gradually.TCAN production went up with the increase in chlorine dosage.At the acidic condition(pH?6),the TCAN production was on the rise when pH was raised.However,the further increase of pH value had a negative effect on the production,because the TCAN was not stable and the alkali catalytic reaction followed by hydrolysis happened when the pH value was more than 7.TCAN production was improved when temperature increased.The TCAN formation process consisted of a series of steps during the reaction between L-tryptophan and disinfectant sodium hypochlorite.The hydrogen ions from amino functional groupswere substituted by the chloride ion produced from hypochlorous acid in the first and second steps.Decarboxylic reaction took place and a carbon dioxide and hydrogen chloride were eliminated in the third step.Hypochlorous acid was oxidized into C?N bond in the fourth step.Cyclic groups were replaced by chloride ion in the next step.TCAN was generated with chloride ion replacing hydrogen ion from methyl in the end after the sixth and seventh steps.The zero valent iron powder had a good effect on TCAN removal.The removal efficiency of TCAN increased gradually with the increase in reaction time,iron powder dosage and reaction temperature.In addition,when initial TCAN concentration increased,the removed amount of TCAN gradually increased and the removal efficiency decreased.TCAN in drinking water was degraded by zero-valent iron powder through the hydrogenolysis and the reaction was in accordance with first-order kinetics.The TCAN removal was improved with the increasing intensity of UV light during the ultraviolet light process.That is because that the higher the intensity of UV light was,the more the photon were and the higher the transformation opportunity of TCAN with higher removal efficiency.With initial concentration rising,the removal was boosted.The temperature had a minor impact on the TCAN removal.The photolysis reaction rate at the early stage and the hydrolysis reaction rate were both enhanced in the alkaline environment so that the removal was improved with the increasing pH.The TCAN degradation process by UV technology conformed to the first-order kinetics.